In probe card metrology applications, it is often necessary or desirable to know the distance between a flat surface (a “primary” or “principal” surface) and another surface to which a probe card is attached (“reference” surface). A common approach employed by many systems is illustrated in FIG. 1. Specifically, FIG. 1 is a simplified diagram illustrating three views of the structural components employed in a typical probe card metrology system. Platforms A and B are connected or rigidly affixed by three or more legs or vertical structural members; the platforms and the legs form a metrology frame to which other components of the metrology system may be attached during use. A z-stage, such as the exemplary wedge driven z-stage, for example, is attached to platform A. The primary surface is typically attached to the top of this stage, while a reference ring or other structural reference component is attached to the bottom side of platform B. Where a ring is used, the top surface of the reference ring is typically designated as the reference plane, and ordinarily supports a probe card to be analyzed. Through linear horizontal translation of wedge C, wedge D may be driven vertically, thereby translating the primary surface relative to the reference surface. In that regard, a linear scale or encoder (labeled “linear encoder” in FIG. 1) may measure displacement of wedge D relative to platform A.
The lower travel limit of the z-stage may be measured (relative to the reference surface) using a depth indicator, for example, as illustrated in FIG. 2. Specifically, FIG. 2 is a simplified diagram illustrating three views of the structural components employed in a probe card metrology system adapted for use with a depth indicator. Such a depth indicator is typically set in a flat bar spanning the reference ring. By first zeroing or calibrating the depth indicator flush with the flat bar, absolute depth of the primary surface can be measured. Similarly, relocating the depth indicator and taking measurements at three points on the primary surface may allow parallelism to be determined. Any non-parallelism may be removed, for example, by adjusting the pitch, roll, or both, of either the z-stage base, platform A, platform B, or some combination thereof. In the embodiment illustrated in FIGS. 1 and 2, the linear encoder is attached between wedge D and platform A; as noted briefly above, this linear encoder may measure displacement of the wedge relative to the platform. Since the starting height is known from the depth indicator measurements, such measurement of the displacement may allow the final height to be determined.
The Abbe principle dictates, however, that displacement at points away from the linear encoder can only be inferred. Any compression or deflection of components above the linear encoder (such as platform B), for example, is not measured, nor is any deflection or deformation of the reference or primary surfaces, such as due to forces exerted by probes during overtravel. Additionally, current technology can provide no information regarding parallelism degradation. Since only one linear encoder is provided, angular displacement cannot be measured absent complicated and time-consuming relocation of the depth indicator and recalibration. Any dimensional changes to the stiffness loop due to temperature or strain, for example, are typically not considered, and can influence measurement results.
In other words, a displacement of 10 μm as measured by the linear encoder in a conventional system does not guarantee uniform, one-dimensional translation of the principal plane relative to the probe card of that 10 μm distance. In that regard, measurement accuracy is a function of the rigidity of the structural components of the system, the trueness of stage travel, the stability of the metrology frame, and other factors which are not taken into account by conventional metrology methods and technologies.